Historically, fibrous structures including those that are used to make sanitary tissue products have been made with softwood fibers and hardwood fibers. For example, softwood fibers have typically made up greater than 20% by weight on a dry fiber basis of through-air-dried fibrous structures. The softwood fibers are longer fibers than the hardwood fibers and they provide greater strength properties to the fibrous structures than do the hardwood fibers. However, softwood fibers typically do not provide the level of softness benefit provided by hardwood fibers.
Trichome fibers have been identified as a good substitute for softwood fibers to provide softness while contributing sufficient strength to a fibrous structure in which they are incorporated. Trichomes are epidermal attachments of a varying shape, structure and/or function of a non-seed portion of a plant. In one example, a trichome is an outgrowth of the epidermis of a non-seed portion of a plant. The outgrowth may extend from an epidermal cell. In one embodiment, the outgrowth is a trichome fiber. The outgrowth may be a hairlike or bristlelike outgrowth from the epidermis of a plant. Trichomes may protect the plant tissues present on a plant. Trichomes may for example protect leaves and stems from attack by other organisms, particularly insects or other foraging animals and/or they may regulate light and/or temperature and/or moisture. They may also produce glands in the forms of scales, different papills and, in roots, often they may function to absorb water and/or moisture.
Individualized trichome fibers can be artificially separated from portions of their host plant. U.S. Pat. No. 7,811,613 describes a process for liberating trichome fibers that includes a milling operation of non-seed portions of a trichome-bearing plant followed by a screening or air classifying step to separate the trichome fibers from other portions (such as the leaves and stems) of the host plant. FIGS. 1 and 2 illustrate such a process. A feedstock of material that includes leaves, stems, and attached trichome fibers is fed into a vacuum system 10 that has two outlets, an upper outlet 12 to collect dust, and a lower outlet 14 to collect further processable material 16. The main purpose of vacuum system 10 is to transport the feedstock and to begin cleaning the feedstock of dirt, dust, and other waste material. Material 16 continues on to a cyclone 18 that also contains an upper outlet 20 for collecting dust, and a lower outlet 22 that feeds a hammermill 24. Magnets 26 and 28 are included to remove any metal that has been introduced into the feedstock via harvesting, transportation, or handling equipment. Hammermill 24 breaks the leaves and stems up into smaller pieces and separates at least some of the trichome fibers from leaves and stems. Material 30 coming out of hammermill 24 is then directly or indirectly routed into a series of air classifiers 32 and 34, as can be seen in FIG. 2. Air Classifier 32 has a waste outlet 33 for collecting the small leaf and stem pieces. Separated trichome fibers are routed to air classifier 34 and then collected for making fibrous structures.
Unfortunately, systems such as those described above produce low yields (mass of feedstock divided by the mass of trichome fibers collected from air classifier 34); for example, up to around 15%. Low yields discourage commercial leveraging of alternative, sustainable resources for fibers that can be used in paper products. One of the reasons for the low yield is that while trichome fibers can be separated from leaves and stems via the hammermill, many of the separated fibers are still tangled with or otherwise associated with the small leaf and stem pieces. These associated trichome fibers then flow out of the waste outlet 33 and never make their way into the fiber collection used for making fibrous structures. The inventors of the present invention have discovered that one or more additional steps of disassociating separated fibers from other plant portions significantly increases trichome fiber yield. The one or more disassociation steps have also been found to remove dirt and other foreign materials to a greater level so that the collected trichome fiber masses are lighter in color and comprise fewer dark specks, making fibrous structures comprising the processed trichome fibers more consumer acceptable.